1. Field of the Invention
This invention relates to methods for growing aluminum-indium-arsenide (AlInAs) layers on indium phosphide (InP)-based devices, and more particularly to growing the AlInAs layer so that it has a high breakdown voltage.
2. Description of the Related Art
Several InP-based devices have been generated in which a significant electric field is developed through an AlInAs layer incorporated into the device. Because of the relatively high fields involved, it is important that the AlInAs layer have a high breakdown voltage. For example, one of the major limitations of InP-based high electron mobility transistors (HEMTs) for power applications has been the gate-to-drain breakdown voltage, which is impressed mostly across an AlInAs layer. The AlInAs layer is referred to as a Schottky layer because it is contacted by a metal gate contact, forming a Schottky junction with a non-linear voltage-current characteristic. InP-based power HEMTs with a gate length of 0.25 microns typically have a breakdown voltage of 7 volts at a current density of 600 mA/mm, with the highest reported breakdown voltage for these parameters being approximately 10 volts (see Matloubian et al., "High power and high efficiency AlInAs/GaInAs on InP HEMTs", IEEE MTT-S Int Microwave Symp. Dig.,1991, pages 721-724). The gate-to-drain breakdown voltage generally increases as the gate length is enlarged and/or the current density is reduced. The relatively low breakdown voltages that have been achieved so far limit the operating voltage of a power HEMT for reliable operation to about 5 volts. Higher power densities and power-added efficiencies could be achieved by increasing the transistor's operating voltage, but this would risk a voltage breakdown.
Electronic devices which employ AlInAs layers are commonly grown by molecular beam epitaxy (MBE). The substrate temperature within the MBE chamber must be controlled, since the ultimate device characteristics will change with different growth temperatures. The absolute InP substrate temperature within the chamber is difficult to measure directly, since the substrate is rotated during the growth cycle. Instead, temperature measurements are commonly made indirectly through a reflection high energy electron diffraction (RHEED) analysis, in which an electron beam is diffracted off the substrate in the presence of a phosphorous flux, and a pattern produced by the diffracted beam is monitored. The substrate temperature is ramped up by an electric heater, positioned to the rear of the substrate, until a characteristic diffracted beam pattern known as the 2.times.4 reconstruction pattern is observed. Although the substrate temperature at which this pattern occurs will generally not be known precisely, it normally occurs at about 540.degree. C. for InP. The 2.times.4 reconstruction temperature then serves as a reference for the MBE growth. The RHEED analysis technique is described in R. Averbeck et al. "Oxide desorption from InP under stabilizing pressures of P.sub.2 or As.sub.4 ", Appl. Phys. Lett. 59(14) 30 Sep. 1991, pages 1732-1734 and in E. Bauer, "Reflection Electron Diffraction", Chapter 15, pages 501-542, Techniques for the Direct Observation of Structure and Imperfections Part II, Interscience Publishers, 1969.
Most of the MBE grown layers of a device such as a HEMT are typically grown at 20.degree. C. below the 2.times.4 reconstruction temperature, or generally at about 520.degree. C. However, the AlInAs Schottky layer is generally grown at a temperature about 40.degree. C. lower, or about 480.degree. C. This is because a relatively high aluminum content (typically about 0.6) is employed to increase the Schottky layer's breakdown voltage, but the high aluminum content also produces a lattice mismatch between the Schottky layer and the InP substrate. The lower growth temperature for the Schottky layer helps to reduce the strain imposed upon it by the lattice mismatch to avoid formation of dislocation, and thus prevents cracking or other damage to the Schottky layer. However, even with a relatively high aluminum content, the Schottky layer's breakdown voltage is still limited as discussed above. With a lower aluminum content of 0.48, an MBE growth temperature of 530.degree. C. was found to be optimum for a device quality HEMT with an AlInAs buffer layer; Georgakilas et al., "A Comprehensive Optimization of InAlAs Molecular Beam Epitaxy for InGaAs/InAlAs HEMT Technology", Journal of the Electrochemical Society, Vol. 140, No. 5, May 1993, pages 1503-1509.
AlInAs Schottky layers have also been grown for HEMTs at a considerably lower temperature region, typically about 150.degree.-200.degree. C. below the 2.times.4 reconstruction temperature. See U.S. Pat. No. 5,084,743 to Mishra et al. The purpose is to obtain a higher resistivity when a pseudo-insulating Schottky layer is desired. However, growth at these lower temperatures results in an even lower breakdown voltage for the Schottky layer.